Ratibor

const elems = document.querySelectorAll('div');

function handler() {
  alert(123);
  elems.forEach(n => n.removeEventListener('click', handler));
}

elems.forEach(n => n.addEventListener('click', handler));

const elems = [...document.querySelectorAll('div')];

document.addEventListener('click', function handler(e) {
  if (elems.some(n => n.contains(e.target))) {
    alert(123);
    document.removeEventListener('click', handler);
  }
});

<select id="country"></select>
<select id="сity"></select>
<p></p>

const data = {
  'Франция': [ 'Париж', 'Марсель', 'Лион' ],
  'США': [ 'Вашингтон', 'Чикаго', 'Фарго' ],
  'Италия': [ 'Рим', 'Милан', 'Неаполь' ],
};

const country = document.querySelector('#country');
const city = document.querySelector('#сity');
const p = document.querySelector('p');

country.addEventListener('change', e => {
  setSelectOptions(city, data[e.target.value]);
});

city.addEventListener('change', () => {
  p.innerText = [ country.value, city.value ].join(', ');
});

setSelectOptions(country, Object.keys(data));


function setSelectOptions(selectEl, optionsData) {
  selectEl.innerHTML = optionsData.map(n => `<option>${n}</option>`).join('');
  selectEl.dispatchEvent(new Event('change'));
}

// validation patterns
const fname =  /^[a-z ]{5,}$/i;
const sname = /^[a-z ]{5,}$/i;
const email = /^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,6})+$/;
const phone = /^\+7[0-9]{10}$/;

const patterns = { 
	name: fname, 
	surname: sname, 
	email, 
	phone, 
	mobilePhone: phone, 
	workPhone: phone 
};

// forms
const forms = [
  { name: 'Ivan Ivanov', email: 'ivan@test.co', phone: '+79212753690' },
  { name: 'III', email: 'ivan@test', phone: '11111' },
  { name: 'Ivan Ivanov', email: 'ivan@test.co', phone: '+79212753690', mobilePhone: '+79212753690'},
  {},
  { name: 'Ivan Ivanov', email: 'ivan@test.co', phone: '+79212753690', mobilePhone: 'фывыафывафыв'},
  { name: 'Ivan Ivanov', email: 'ivan@test.co', phone: '+79212753690', mobilePhone: '+79212753690', workPhone: 'asfdadsfd', surname: ''},
  { surname: 'Ivan Ivanov', mobilePhone: 'фывафыва', name: 'Ivan Ivanov', email: 'ivan@test.co', phone: '+79212753690', workPhone: '+79212753690'
  }
];

validateAll(forms, patterns);

function validateAll(forms, patterns) {
	return forms.map( form => validate(form, patterns) );
}


function validate(form, patterns) {
	const keys = Object.keys(form);
	const errors = [];
	
	if(!keys.length) return {target: {}, errors: ['empty form is provided']};
	
	keys.forEach(key => {
		if( !patterns[key].test(form[key]) ) {
			errors.push(`${key} value is invalid`);
		}
	});
	
	return {
		target: JSON.stringify(form),
		errors
	};
}

var keys = ['2', '4', 'R', '2', 'd', '2'];
var result = keys.reduce(function(accum, current){
  return !accum ? current : '' + accum + current;
});
console.log(result);

hadleKeyFactory = function() {
    var result = '';
    return function(key) {
        return (result += key);
    } 
}

var handleKey = new hadleKeyFactory();
handleKey('a');
// output 'a'
handleKey('b')
// output 'ab'

var keys = ['2', '4', 'R', '2', 'd', '2'];

function glue(a) {
	var s = '', i = 0;
	while (s.length !== a.length) {
		s += a[i++];
	}
	return s;
}